This invention relates generally to electronic devices and, more particularly, to a radio frequency receiver for mobile communication.
In a code division multiple access (CDMA) wireless phone system, information is transmitted over radio frequency (RF) communication channels that are established between a base station and one or more mobile stations. Each mobile station includes a radio frequency receiver circuitry used to select signals in a desired communication channel, and to convert the selected radio frequency signals to baseband digital signals for further signal processing.
Mobile station receivers may be based on a number of different technologies. For example, heterodyne signal reception technology may be used. In a heterodyne receiver, a received RF signal is applied to an image-reject filter and then to a low-noise amplifier (LNA). The resulting signal is mixed with a local oscillator to produce a signal having an intermediate frequency (IF) carrier. The IF signal is then passes through an IF filter which can be used to suppress out-of-channel interference, thus performing channel selection. The transmitted information may then be extracted from the selected channel. Heterodyne receiver designs may struggle with an undesirable tradeoff between image rejection and interferer channel suppression.
The use of direct conversion receivers has also been proposed as a solution to some of these issues. In a direct conversion receiver, also called a zero-IF receiver, the signal of interest is converted directly to a zero frequency carrier rather than to an intermediate frequency carrier. Direct conversion receivers are further described in Design Considerations for Direct-Conversion Receivers, Behzad Razavi, IEEE Transactions on Circuits and Systems: Analog and Digital Signal Processing, Vol. 44, No. Jun. 6, 1997 [Razavi]. Razavi asserts that direct conversion receivers have several advantages over heterodyne receivers including lower cost, configuration flexibility, circumvention of the heterodyne image rejection problem and easier monolithic integration.
Razavi notes that previous attempts to use direct conversion technology have met with limited success.
In general, in one aspect, the invention features a signal reception method. The method includes mixing a received RF signal and a local oscillator signal at a signal image rejection mixer to form a frequency down converted signal. The received RF signal includes information data that modulates a RF transmission carrier. The down converted signal formed from the received RF signal is a modulated non-zero low frequency carrier referred to as a low frequency carrier desired signal. A digital representation of the low frequency carrier desired signal is then formed. This signal reception method is called a pseudo direct conversion (PDC) architecture.
Implementations may include one or more of the following features: a local oscillator signal may be selected such that, during down-conversion of the received RF signal, interference signals are shifted or folded onto frequencies that are lower or greater than the frequencies occupied by the low frequency carrier desired signal. The down converted signal may then be low-pass or band-pass filtered to attenuate frequencies outside the spectrum of the low frequency carrier desired signal. The low frequency carrier desired signal may be amplified and digitally sampled at an analog-to-digital converter to form a digital representation of the desired signal.
Implementations may also include a quadrature frequency down-converter consisting of two image rejection mixers, a power splitter in signal path, and a xcfx80/2 phase shifter implemented in a path between the local oscillator and one of the image rejection mixers. The received RF signal is split into two portions. A first portion is directly mixed with the local oscillator signal at the first image rejection mixer to form an I channel low frequency carrier desired signal and to form frequency reduced interferers and/or spectrum folded interferers. A second portion is mixed with a xcfx80/2 phase shifted version of the local oscillator signal to form a Q channel low frequency carrier desired signal and to form frequency reduced interferers and/or spectrum folded interferers. The resultant desired signals (and interferers) in the I and Q channels have the same carrier frequency and magnitude, however xcfx80/2 phase shift exists between the two desired signals.
Implementations may also include one or more of following features: the down-converted desired signals in the I and Q channels are filtered by low-pass filters or band-pass filters. Also, the I and Q channels are amplified through alternative current (AC) coupling approach. The signal in each of the I and Q channels is sampled by an analog-to-digital converter (ADC) in its respective channel. The sampling rate of these ADCs should be 2xc3x97fd+BW where fd is the low carrier frequency of the desired signal and BW is the bandwidth of the desired signal. The digitized I and Q channel desired signals are low-pass or band-pass filtered and are summed together in a complex form Ixc2x1jQ where j={square root over (xe2x88x921)} and sign +/xe2x88x92 depends whether on the down-conversion condition of the received signal frequency is higher or lower than the local oscillator frequency. Thus, the desired signal can be easily selected from interferers by using a digital band-pass filter.
In general, in another aspect, the invention features a wireless receiving apparatus. The apparatus includes an antenna at which a radio frequency (RF) signal is received. The received RF signal is mixed with a local oscillator signal at a signal image rejection mixer to form a frequency down converted signal. The received signal includes information data that modulates an RF transmission carrier. The down converted signal formed from the received RF signal is a modulated non-zero low frequency carrier referred to as a low frequency carrier desired signal. Using a digital-to-analog converter, a digital representation of the low frequency carrier desired signal is then formed. The receiver based on this signal reception method is called a pseudo direct conversion receiver (PDCR) architecture.
Implementations of the PDCR may include one or more of the following advantages. The pseudo-direct conversion (PDC) receiver architecture reduces the need for IF surface acoustic wave (SAW) filters in the RF section of the receiver. Low-pass filters (LPFs) or band-pass filters (BPFs) used in the PDC receiver may be designed and implemented in a monolithic integration circuit (IC) chip. Receiver saturation due to DC coupling between circuits can be reduced or eliminated through the use of AC coupling between circuit elements in the PDC receiver. Residual DC offsets may be removed using digital domain processing. Even-order distortion effects can be reduced or eliminated. The PDC receiver may be designed so that interferer signals generated by second order distortion produced by the nonlinearity of low noise amplification and down-conversion are located outside the spectrum of the desired signal; thus, there is no direct interfering to the desired signal. Distortion due to flicker noise may be maintained outside the frequency range of interest. Implementations may not need to separate I and Q channels, thereby no I/Q mismatch issue arises.
Implementations may also include one or more of the following advantages. PDC receiver architectures may be used with a variety of wireless mobile communication systems including third generation (3G CDMA) which includes direct spread wideband CDMA (WCDMA), and multi-carrier CDMA (MC-DMA). For example, a PDC receiver can operate in either the WCDMA mode or the MC-CDMA mode by altering the program code that is executed by a digital filter. In the WCDMA mode, a 3.84 MHz single pass band filter algorithm is executed, and in the MC-CDMA mode, a multiple pass band filter bank with each filter having a band spacing of 1.25 MHz and bandwidth of 1.23 MHz is executed.
Implementations may have other and/or alternative advantages, as will be clear from the description and claims that follow.